YEASTS, MOLDS AND TORUL/2 51 



perfectly dry, the paper is cut away and the block is placed in a 

 suitable vessel (a Stender dish or a deep, narrow Petri dish will 

 answer very well). The block and vessel are then sterilized in 

 the hot-air sterilizer for one hour. 



4. Pour enough distilled water around the gypsum 

 block to submerge about one-half of it. 



5. Set aside in a cool, dark place for 3 or 4 days. 



6. Examine a small portion of the film on the surface 

 of the gypsum under the microscope in water. 



NOTE. Under very favorable conditions, and in the pres- 

 ence of plenty of oxygen, yeasts will undergo spore-formation. 

 The great porosity of the gypsum block, which admits free com- 

 munication with the water, and the fact that the surface of the 

 block is exposed to the air, offer most favorable conditions for 

 spore-formation, which takes place in 3 or 4 days. 



EXERCISE II. THE STUDY OF THE GERMINATION OF 

 SPORES 



The germination of spores may be studied from spores 

 (conidia) of mold fungi. Although somewhat different 

 from the germination of bacterial spores, mold spores 

 illustrate the general process very well. 



1. Make a suspension of spores of Penicillium glau- 

 cum (common green mold) in broth. 



2. Flame carefully a hollow-ground slide so as to 

 exclude all other organisms, and paint vaselin around 

 the hollow so as to form a complete ring. 



3. Take a loopful of the spore suspension and place it 

 on a clean flamed cover-glass. 



4. Invert over the prepared hollow-ground slide, and 

 gently press the edges on the vaselin so as to exclude all 

 communication with the air. This prevents the hang- 

 ing-drop from drying out, and also the invasion of for- 

 eign organisms. 



